This invention relates in general to sewing machines and in particular to a new and useful buttonhole sewing machine clamping device which effects the stretching of the workpiece during the clamping operation.
The invention relates in particular to a work clamp for buttonhole sewing machines by which the work is tensioned so that the buttonhole sewing area is smooth, flat and taut during the sewing operation.
A work clamp of this kind has become known through U.S. Pat. No. 2,187,235. This work clamp comprises an oval hold-down arranged device at a pivotably mounted carrying lever and resiliently tensioned against a support plate and having an opening corresponding to the buttonhole form, in which opening a tubular ram connected with the carrying lever is movable relative to the hold-down device. The support plate contains an opening matching the tubular ram, for passage of the buttonhole cutter or of the needle. As the hold-down device descends, the work placed on the support plate under the raised hold-down device is first pressed against the support plate around the cutting and sewing area and is clamped tight. In the further course of the downward movement, the tubular ram pushes the work into the opening in the support plate and in so doing tensions the sewing area to all sides like the hide of a drum.
With this work clamp, no separate drive means for tensioning the work in the sewing area are required other than the lowering device for the carrying lever, but the disadvantages of this type of tensioning is the fold formation as the material is being pulled into the bore by the ram. When working with a thick cloth, damage due to pinching is to be expected, while the tensioning of thin fabrics is not possible at all.
Through German Pat. No. 886,242 another work clamp is known, by which the work is first pressed against the work support plate completely by the work clamp frame provided with two clamping ribs, the support plate including the stitch plate. Cut into the work support plate is a groove which runs parallel to the longitudinal axis of the buttonhole and into which the work is pressed by a tensioning rod arranged movable in height next to one clamping rib so as to be tensioned in the buttonhole sewing area after it has been clamped to the support plate by the work clamping frame. The tensioning rod can be actuated for the tensioning of the work either manually by actuation of a lever or mechanically through an additional lever system which becomes operative as the sewing machine is set in motion. Pressing the work into the groove in the support plate not only involves the danger of a deformation that remains visible, but because of the arrangement of the tensioning rod on one side of the later buttonhole slit right next to one clamping rib, a transverse pull results in the fabric, which may very easily lead to a distortion of the individual threads of the fabric web in this area between the clamping ribs of the work clamping frame and thereby may cause or intensify rippling of the material, which tends to curl anyway due to the accumulation of stitches, around the buttonhole.